A water absorbent resin absorbs a large amount of aqueous liquid whose weight is several to several hundred times as heavy as its own weight. As such, the water absorbent resin is widely used for various uses in sanitary materials (e.g., disposable diapers, sanitary napkins, and adult incontinence pads), soil water-holding agents, and the like, and are massively produced and consumed. Such a water absorbent resin (referred to also as “highly water absorbent resin” or “water absorbent polymer”) is already publicly known because it is described, for example, in Japanese Industrial Standards (JIS) K7223-1996 and introduced in a large number of commercially-available reference books.
In recent years, especially for use in sanitary articles such as disposable diapers, sanitary napkins, and adult incontinence pads, the amount of water absorbent resin to be used and the amount of pulp fiber to be used tend to be increased and decreased, respectively, for the purpose of thinning the products. This makes it necessary for the water absorbent resin to take over the conventional functions of the pulp to permeate and disperse a liquid in an absorbent core. Proposed as publicly-known indices for evaluating the quality of such functions are the absorbency against pressure and liquid permeability of a water absorbent resin. Therefore, there is a demand for a water absorbent resin high in absorbency against pressure and liquid permeability. Meanwhile, the tendency toward thinning the products leads to an increase in amount of water absorbent resin to be used for each sanitary article, thereby causing a rise in demand for a low-cost water absorbent resin.
In general, a water absorbent resin is provided in powder form by drying and pulverizing a hydrogel polymer obtained by performing aqueous polymerization of a hydrophilic unsaturated monomer. The hydrogel polymer is obtained as a clump or an agglomerate of hydrogel particles. Generally, the hydrogel polymer is roughly crushed by a crusher such as a kneader, a meat chopper, or the like so as to have a particle diameter of approximately 1 mm to 10 mm. Further, the hydrogel thus roughly crushed is dried so as to have a solid content of approximately 95 wt %, and then pulverized by a pulverizer so as to have a weight-average particle diameter of not less than 300 μm to not more than 600 μm.
In the pulverization step after the drying, there occur particles whose particle diameter deviates from the desired particle diameter range. Accordingly, the polymer thus pulverized after the drying is sieved by a classifier so as to have a particle diameter falling within the desired particle diameter range. Thus obtained is a particulate water absorbent resin. Although there are variations in use, it is preferable that the particle diameter of a particulate water absorbent resin for use in sanitary articles fall within a range of not less than 150 μm to less than 850 μm.
It should be noted here that examples of a method for drying the hydrogel include a method for drying the hydrogel with the hydrogel being left at rest, a method for drying the hydrogel with the hydrogel being stirred, a method for drying the hydrogel with the hydrogel being shaken, a method for drying the hydrogel with the hydrogel flowing, and a method for drying the hydrogel with airflow (e.g., see Patent Documents 1 to 10).
Among them, Patent Document 1 discloses a method for producing a particulate water absorbent resin by drying and pulverizing a hydrous polymer immediately after polymerization and, for example, by further drying and pulverizing the particulate hydrous polymer thus pulverized. Further indicated as a method for performing the second drying is a method, such as a method for performing drying by stirring, a method for performing drying in a fluidized bed, or a method for performing drying with airflow, by which the material is moved so as to make sufficient contact with hot air or a heat transfer surface.
Further, in general, a water absorbent resin is produced through drying and surface crosslinking of a hydrogel polymer obtained by performing polymerization of an aqueous solution containing a hydrophilic monomer and a cross-linking agent. The hydrogel polymer is obtained as a clump or as an agglomerate of hydrogel particles. Generally, the hydrogel polymer is roughly crushed by a crusher such as a kneader, a meat chopper, or the like so as to have a particle diameter of approximately 1 mm to 10 mm. Further, the hydrogel thus roughly crushed is dried so as to have a solid content of approximately 95 wt %.
In the pulverization step after the drying, the hydrogel thus dried is pulverized by a pulverizer so as to have a weight-average particle diameter of not less than 300 μm to not more than 600 μm. At this time, there occur particles whose particle diameter deviates from the desired particle diameter range. Accordingly, the polymer thus pulverized after the drying is sieved by a classifier so as to have a particle diameter falling within the desired particle diameter range. Thus obtained is a particulate water absorbent resin. Although there are variations in use, it is preferable that the particle diameter of a particulate water absorbent resin for use in sanitary articles fall within a range of not less than 150 μm to less than 850 μm. After a surface cross-linking step, the water absorbent resin obtains properties, such as absorbency against pressure and liquid permeability, suitable for use in sanitary agents (sanitary materials) and the like. In the customary sense, the surface cross-linking step is a step of providing a highly cross-linked layer near the surface by bringing the water absorbent resin into reaction with a surface cross-linking agent or a polymerizable monomer.
Examples of surface cross-linking techniques hitherto discussed include: a surface cross-linking technique that involves concomitant use of a surface cross-linking agent (Patent Document 11); a surface cross-linking technique that involves an apparatus for mixing a water absorbent resin and a surface cross-linking agent (Patent Document 12); a surface cross-linking technique that involves a heating apparatus in which a water absorbent resin and a surface cross-linking agent are brought into reaction (Patent Document 13); a surface cross-linking technique that involves control of a rise in heating temperature at which a water absorbent resin and a surface cross-linking agent are brought into reaction (Patent Document 14); and a surface cross-linking technique that involves a process for surface crosslinking of a water absorbent resin with a high moisture content (Patent Document 15). Further, unlike the normal surface cross-linking treatment, there are such examples where a water absorbent resin is modified by heating without use of a surface cross-linking agent (Patent Documents 16 and 17).
Further, there have been proposed various surface cross-linking agents known as, e.g., an oxazoline compound (Patent Document 18), a vinylether compound (Patent Document 19), an epoxy compound (Patent Document 20), an oxetane compound (Patent Document 21), a polyvalent alcohol compound (Patent Document 22), a polyamide polyamine-epihalo adduct (Patent Documents 23 and 24), a hydroxyacrylamide compound (Patent Document 25), an oxazolidinone compound (Patent Document 26), a bis- or poly-oxazolidinone compound (Patent Document 27), a 2-oxotetrahydro-1,3-oxazolidine compound (Patent Document 28), and an alkylene carbonate compound (Patent Document 29). Further known are a technique for polymerization and surface crosslinking of monomers (Patent Document 30) and techniques for radical crosslinking with persulfate and the like (Patent Documents 31 and 32).
Furthermore, there have been proposed techniques that involve concomitant use of additives in mixing of surface cross-linking agents, and known as the additives are, e.g., water-soluble cations such as aluminum salt (Patent Documents 33 and 34), an alkali (Patent Document 35), and an organic acids or an inorganic acid (Patent Document 36). Further known is a technique that involves use of a particular mixer as a mixer for a surface cross-linking agent (Patent Document 37).
Further proposed are a technique for performing surface crosslinking twice in a heating step (Patent Document 38), a technique that involves use of a plurality of heat treatment apparatuses in a heating step (Patent Document 39), and techniques that involve heating of water absorbent resins before surface crosslinking (Patent Documents 40 and 41).
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 051967/2004 (Tokukai 2004-051967; published on Feb. 19, 2004)
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 212204/2002 (Tokukai 2002-212204; published on Jul. 31, 2002)
[Patent Document 3]
Japanese Unexamined Patent Application Publication No. 240914/1999 (Tokukaihei 11-240914; published on Sep. 7, 1999)
[Patent Document 4]
Japanese Translation of PCT Patent Application Publication No. 506363/1996 (Tokuhyohei 8-506363) published on Jul. 9, 1996)
[Patent Document 5]
Japanese Unexamined Patent Application Publication No. 018222/2001 (Tokukai 2001-018222; published on Jan. 23, 2001)
[Patent Document 6]
Japanese Translation of PCT Patent Application Publication No. 511488/2003 (Tokuhyo 2003-511488) published on Mar. 25, 2003)
[Patent Document 7]
Japanese Unexamined Patent Application Publication No. 212215/2000 (Tokukai 2000-212215; published on Aug. 2, 2000)
[Patent Document 8]
International Publication No. WO 2006/100300 (published on Sep. 28, 2006)
[Patent Document 9]
U.S. Unexamined Patent Application Publication No. 2008/0021140 (published on Jan. 24, 2008)
[Patent Document 10]
International Publication No. WO 2008/087114 (published on Aug. 4, 2008)
[Patent Document 11]
U.S. Pat. No. 5,422,405 (published on Jun. 6, 1995)
[Patent Document 12]
Japanese Unexamined Patent Application Publication No. 214734/1992 (Tokukaihei 4-214734; published on Aug. 5, 1992)
[Patent Document 13]
Japanese Unexamined Patent Application Publication No. 352941/2004 (Tokukai 2004-352941; published on Dec. 16, 2004)
[Patent Document 14]
U.S. Pat. No. 6,514,615 (published on Feb. 4, 2003)
[Patent Document 15]
U.S. Pat. No. 6,875,511 (published on Apr. 5, 2005)
[Patent Document 16]
U.S. Pat. No. 5,206,205 (published on Apr. 27, 1993) [corresponding to Japanese Unexamined Patent Application Publication 194762/1993 (Tokukaihei 5-194762; published on Aug. 3, 1993)]
[Patent Document 17]
European Patent No. 0603292 (published on Jun. 29, 1994)
[Patent Document 18]
U.S. Pat. No. 6,297,319 (published on Aug. 2, 2001)
[Patent Document 19]
U.S. Pat. No. 6,372,852 (published on Apr. 16, 2002)
[Patent Document 20]
U.S. Pat. No. 6,265,488 (published on Jul. 24, 2001)
[Patent Document 21]
U.S. Pat. No. 6,809,158 (published on Aug. 26, 2004)
[Patent Document 22]
U.S. Pat. No. 4,734,478 (published on Mar. 29, 1988)
[Patent Document 23]
U.S. Pat. No. 4,755,562 (published on Jul. 5, 1988)
[Patent Document 24]
U.S. Pat. No. 4,824,901 (published on Apr. 25, 1989)
[Patent Document 25]
U.S. Pat. No. 6,239,230 (published on Mar. 29, 2001)
[Patent Document 26]
U.S. Pat. No. 6,559,239 (published on Mar. 6, 2003)
[Patent Document 27]
U.S. Pat. No. 6,472,478 (published on Oct. 29, 2002)
[Patent Document 28]
U.S. Pat. No. 6,657,015 (published on Dec. 2, 2003)
[Patent Document 29]
U.S. Pat. No. 5,672,633 (published on Sep. 30, 1997)
[Patent Document 30]
U.S. Unexamined Patent Application Publication No. 2005/48221 (published on Mar. 3, 2005)
[Patent Document 31]
U.S. Pat. No. 4,783,510 (published on Nov. 8, 1988)
[Patent Document 32]
European Patent No. 1824910 (published on Aug. 29, 2007)
[Patent Document 33]
U.S. Pat. No. 6,605,673 (published on Aug. 12, 2003)
[Patent Document 34]
U.S. Pat. No. 6,620,899 (published on Sep. 16, 2003)
[Patent Document 35]
U.S. Unexamined Patent Application Publication No. 2004/106745 (published on Jun. 3, 2004)
[Patent Document 36]
U.S. Pat. No. 5,610,208 (published on Mar. 11, 1997)
[Patent Document 37]
U.S. Pat. No. 6,071,976 (published on Jun. 6, 2000)
[Patent Document 38]
U.S. Pat. No. 5,672,633 (published on Sep. 30, 1997)
[Patent Document 39]
U.S. Unexamined Patent Application Publication No. 2007/0149760 (published on Jun. 28, 2007)
[Patent Document 40]
Japanese Unexamined Patent Application Publication No. 242709/1995 (Tokukaihei 7-242709; published on Sep. 19, 1995)
[Patent Document 41]
Japanese Unexamined Patent Application Publication No. 224204/1995 (Tokukaihei 7-224204; published on Aug. 22, 1995)